Air operable control drives are equipped with air failure brakes which are intended to hold the drive in position in the event of failure of the air supply. Prior art mechanical friction brakes, such as drum or disc brakes, require high forces to actuate and hold the drive in position. This requires a large air cylinder to create sufficient force when working with relatively low air pressure, e.g., 100 psi, which is normally used in such devices.
Because of the need for high actuating forces and a large cylinder required thereby, it has become desirable to develop a brake which includes some form of mechanical advantage which can reduce the actuating force required, and which includes some form of positive engagement so that friction force alone is not relied on to hold the control in position.